Registration of frameworks with set but empty ID should not be allowed

Mesos currently allows frameworks to register with a set, but empty ID. Internally this is treated identically to the case of registration without a set ID, and quite some code exists to support this.

We should check whether we really need to provide this level of flexibility. It not only complicates the implementation, but also the API which both leads to conceptually harder to grasp code (which tends to be error prone). Ideally we should reject a set but empty FrameworkID.